1. Field of the Invention
The present invention relates to a torque detecting apparatus and, more particularly, to an improved torque detecting apparatus which is capable of noncontactingly detecting the torque transmitted through a rotary magnetic material as magnetostriction.
2. Description of the Related Art
In various kinds of rotating drive mechanisms there is a need to measure torque simply and accurately because such measurement is exceedingly useful in a diverse range of industrial field for analyzing drive mechanisms and for obtaining a better understanding of their operating condition.
Rotary drive mechanisms are used as prime movers in virtually every sector of industry, the most common types thereof being automobile engines, electric motors of electric cars and industrial motors.
In accurately obtaining and analyzing the operating condition of such mechanisms, it is as important to be able to accurately determine transmitted torque as it is to determine the number of revolutions. Measurement of torque is particularly important in the case of automobile engines because by measuring the torque at the engine and at the transmission, propeller shaft, differential gear and other points of the drive system it is possible to control the ignition timing, the amount of fuel injection, the timing for transmission shift, the gear ratio, etc. As a result of the optimum control of these factors, it is possible to improve fuel efficiency, driving characteristics, etc.
In the case of industrial motors, accurate torque measurement can also provide data for optimizing control and diagnosis of rotary drive systems, thereby improving energy efficiency and driving characteristics.
For these purposes, various kinds of torque detecting apparatus have conventionally been proposed, one of them being an apparatus for noncontactingly measuring the torque transmitted through a rotary magnetic material as magnetostriction (U.S. Pat. No. 2,912,642).
It has already been known that when torque is transmitted through a rotary drive system, strain is produced in the rotary members such as a rotating shaft, flywheel and a clutch disc in proportion to the transmitted torque (U.S. Pat. No. 4,589,290, U.S. Pat. No. 4,697,460). Furthermore, a technique been proposed of noncontactingly measuring transmitted torque by detecting with a magnetic sesor the magnetostriction of a rotary magnetic material, which is a part of a rotary member for transmitting torque and is made of a ferromagnetic material (U.S. application No. 39,390).
FIGS. 12 and 13 show an example of a torque transmission mechanism of a vehicle engine provided with a magnetic sensor 12 of a torque detecting apparatus. FIG. 12 is a schematic side elevational view of the magnetic sensor 12, and FIG. 13 is a schematic sectional view of the magnetic sensor 12 shown in FIG. 12, taken along the line XIII--XIII.
As is known, the torque produced in the engine is transmitted to a rotary flywheel (not shown) through a torque transmitting shaft 10, and is transmitted to the transmission through a clutch disc which comes into frictional contact with the flywheel. When torque is transmitted in this manner, anisotropy of strain e which is proportional to the magnitude of the transmitted torque is generated on the torque transmitting shaft 10 and the rotary discs such as the clutch disc and the flywheel. If the torque transmission member is made of a ferromagnetic material, it is possible to measure the transmitted engine torque by magnetically detecting the magnitude of the generated anisotropy of strain by utilizing the magnetostrictive effect.
In the torque measuring apparatus, therefore, in order to make the rotary member to which torque is transmitted a rotary magnetic material, the torque transmitting shaft 10 or the flywheel themselves is made of a ferromagnetic material, or a ferromagnetic material is attached to the surface of the torque transmitting shaft 10 or the flywheel, and a magnetic sensor 12 is opposed to the rotary magnetic material formed in this manner with a predetermined space therebetween.
The magnetic sensor 12 is composed of a U-shaped excitation core 14 which is disposed in parallel to the torque transmitting shaft 10, and a U-shaped detection core 18 which is disposed inside the excitation core 14 such as to be orthogonal thereto, with an excitation coil 16 wound around the excitation core 14, and a detection coil 20 wound around the detection core 18.
FIG. 14 is a block diagram of the torque detecting apparatus. To the excitiation coil 16 a sine-wave voltage is applied from an AC power source 22 for alternating magnetization of the torque transmitting shaft 10, which is opposed to the magnetic sensor 12.
When torque is transmitted through the torque transmitting shaft 10, stress is produced in the torque transmitting shaft 10 and a magnetic flux component is generated in the direction orthogonal to the direction of excitation by virtue of the magnetostrictive effect.
The magnetic flux component is detected by the detection coil 20 as an induced voltage. The induced voltage is amplified by an alternating amplifier 24 and is thereafter rectified by a detector 26. This rectified signal S is output as a torque detection signal.
In this way, the torque detecting apparatus enables simple and accurate measurements of the transmitted torque in various kinds of rotating drive mechanisms, thereby analyzing drive mechanisms and obtaining a better understanding of their operating condition.
The conventional torque apparatus, however, is disadvantageous in that it is susceptible to disturbing magnetic fields for reasons to be given below, and particularly when a pulsating magnetic field is applied, the magnetized state of the rotary member is forced to change, thereby making the accurate measurement of transmitted torque impossible.
(a) FIG. 15 shows the magnetization characteristics of a rotary magnetic material. The rotary magnetic material is in the state of magnetization 0 (hereafter referred to as "zero magnetization"), as is indicated by the point B. When a pulsating disturbing magnetic field is applied to the rotary magnetic material in this state, the magnetized state is shifted to a residual magnetization state, as is indicated by the point A.
In the conventional detecting apparatus, therefore, after a pulsating disturbing magnetic field is generated, the magnetized state changes, which disadvantageously causes a change in the torque detection output. In other words, the conventional detecting apparatus cannot detect the torque accurately if there is much magnetic noise.
In particular, when such a conventional detecting apparatus is mounted on a vehicle engine, the rotary magnetic material is often magnetized due to a pulsating disturbing magnetic field irregularly generated from a solenoid valve or a spark plug. In this case, the conventional apparatus, which detects transmitted torque by utilizing a magnetostrictive effect, cannot detect the torque with high accuracy and good reproducibility, because the detection torque output inconveniently contains a comparatively large error component.
(b) When a nonuniform disturbing magnetic field is applied to the rotary magnetic material, irregular magnetization is produced so that the magnetized state becomes nonuniform at each part of the rotary magnetic material.
In this case, as shown in FIG. 10, the fluctuation width of the offset signal (detection signal S output when the transmitted torque is zero) in the circumferential direction of the rotary magnetic material becomes very large and, particularly, since the magnetized part produces a spike wave form, the detection signal S inconveniently changes even when the transmitted torque is constant.
Especially, in the case where the fluctuation of the offset output in the circumferential direction of the rotary magnetic material is large, when the rotary magnetic material is rotated, the detection output S varies as if the torque were transmitted even when the transmitted torque is zero. An effective countermeasure has therefore been demanded.
Recently, the measurement of transmitted torque with high accuracy and high responsiveness has increasingly been required for the purpose of the optimum control of various rotary systems such as an engine and a transmission. To meet such demand, development of an apparatus has been demanded which is capable of solving the above-described problems (a) and (b), and detecting the transmitted torque of a rotary magnetic material from a low rotation range to a high rotation range with high responsiveness, high accuracy and good reproducibility.